A thermoelectric cooler (TEC) is a semiconductor device that functions as a heat pump by taking advantage of a phenomenon known as the Peltier effect. In a system having a junction between two dissimilar metals, an electrical current passing through the junction (in a closed circuit) also causes heat to be transferred across the junction. Thus, when current is applied to a thermoelectric cooler, heat moves through the module from one side to the other in proportion to the applied current. One module face will be cooled while the other is simultaneously heated. This phenomenon is fully reversible; with a switch in the polarity of the applied current, heat moves in the opposite direction. Thus, the same module can function as both a heater and a cooler, permitting very precise temperature stabilization. Thermoelectric cooler temperature control is particularly effective for tasks requiring precise temperature control, such as laser diode cooling and temperature control for other sensitive electronic devices.
In order to maintain a stable temperature for a particular device, feedback control systems are frequently implemented. In a feedback control system, one measures the temperature of the device of interest, compares that measured temperature to a desired set point temperature, and appropriately heats or cools the device in an attempt to bring the device temperature towards the set point. FIG. 1 illustrates a simplified block diagram of a feedback temperature control system using a thermoelectric cooler.
In the system illustrated, some thermal load 100, e.g., a device whose temperature is to be controlled such as a laser diode, is thermally coupled to a thermoelectric cooler 105, which in turn is typically coupled to a heat sink 115. In this example, thermal load 100 is coupled to the “cool” side of thermoelectric cooler 105 and so the primary mechanism by which the temperature of thermal load 100 is controlled is by varying the degree to which it is cooled. A temperature sensor 110, typically a thermistor, provides a mechanism for the feedback temperature control system to measure the temperature of thermal load 100.
Temperature controller 120 provides the power needed to operate thermoelectric cooler 105 based on a temperature set point value 170 and information about the temperature of thermal load 100. A signal proportional to the temperature of thermal load 100 is provided to sensor interface 160, which typically includes an amplifier for conditioning the signal from temperature sensor 110. The conditioned signal is fed along with temperature set point 170 into a difference amplifier 130, which produces a signal that in turn operates a control function 140. Power driver 150 provides power to thermoelectric cooler module 105 based on signals from control function 140.
Many contemporary electronic systems are designed to operate from a single positive voltage supply. While single-supply operation presents no serious difficulties if one wishes to operate a thermoelectric cooler in a heat-only or cool-only mode, it does make variable heat/cool operation more complex. FIG. 2 illustrates a simplified schematic diagram of an H-bridge circuit topology, which can provide bipolar drive to a thermoelectric cooler 200 or other load while still operating from a single supply voltage. H-bridge circuit 210 is a type of current control circuit that includes switches 220, 230, 240, and 250 arranged with respect to the load device 200 a shown. In this example, the differential output of an operational amplifier 260 provides control signals to the switches. The rest of the temperature control system is not shown.
In some implementations, the switches of H-bridge circuit 210 are controlled using pulse-width modulated (PWM) signals. Such control typically allows the switches to be fully on (a 100% duty cycle for a given switching frequency) or fully off (0% duty cycle) and thereby limits power-dissipation that can occur when the switches are partially on, e.g., a condition not uncommon among linearly controlled H-bridges. Because PWM control can reduce power dissipation, it can permit the use of smaller transistors, which is a common design advantage. However, PWM operation typically creates large current spikes in the system. Many systems that use H-bridges, especially laser-based communication systems, are particularly sensitive to the noise generated by current fluctuations and the high-frequency switching associated with PWM control. Linear control of the H-bridge can avoid switching noise by controlling the on state of the switching devices precisely to the level needed to perform the right amount of cooling or heating. However, precisely controlling a number of transistors in an H-bridge configuration presents several challenges. So-called “dead-zones” between heating and cooling modes must be accommodated and the larger H-bridge devices may increase overall system cost, size, and/or heat budgets.
In the circuit of FIG. 2, a positive output signal (V+>V−) from operational amplifier 260 will cause transistor 240 and transistor 230 to turn on in a proportional manner, causing current to flow from the +5V rail to the emitter of transistor 240, through the TEC module 200, and into the emitter of transistor 230 before returning to ground. In the case of a negative output signal from operational amplifier 260, transistors 220 and 250 will turn on, and will result in a current flowing in the opposite direction through the TEC 200. The amount of voltage applied across TEC 200, and consequently the amount of current flowing through it, will be proportional to output voltage of operational amplifier 260. An important feature of this circuit is that, in general, at no time do all the transistors turn on, which would effectively short the power supply to ground. As effective as this implementation is, it does suffer from certain shortcomings.
Because the transistors are being used in emitter-follower configurations, there will typically be voltage drop (e.g., approximately 0.7 V) from the base to the emitter of each active device. This voltage drop will limit the maximum output voltage swing to considerably less than the 5V available at the supply. Another limitation of this circuit is that it requires high drive currents from amplifier 260 because of the finite DC current gain (HFE) of the transistors used in the bridge. H-bridge circuit 210 may also oscillate at RF frequencies, depending on the characteristics of the transistors used. One common solution to address these problems is use of a second H-bridge (formed across a small load resistor), which is used to control an H-bridge formed across the thermoelectric cooler device. While somewhat effective, this approach requires many additional components, e.g., four additional switches. Moreover, the added complexity of the circuit can complicate efforts to achieve better system performance such as linearized driver response, gain stabilization, and current limiting.
Accordingly, it is desirable to have a H-bridge circuits and control circuits for H-bridge circuits that provide continuous linear operation and current limiting while reducing the need for additional switches and reducing power losses of the non-TEC components in the system.